Prior art valves for cisterns may be conveniently divided into two broad categories. The first category is mechanical leverage valves having a valve seat with a circular orifice therein, and a disc which is moved by a float arm to open or occlude the circular orifice. A float attached to the float arm provides a buoyant force which is magnified by the mechanical advantage available because of the length of the float arm. Such an arrangement is relatively simple, very reliable, and has been in use for many years.
However, in recent times the average width of cisterns has been reduced because of a reduction in the volume of water used to flush lavatory pans. Since smaller volumes of water are required, corresponding reductions in the size of the cisterns used to hold the flushing water have also been achieved. As a consequence, the length of the float arm previously able to be used, cannot now be used for such cisterns. As a result, the closing force of the mechanical inlet valve referred to above is reduced. This renders this type of valve unsuitable for use in those areas having high water pressures.
As a consequence of this development, hydraulic inlet valves are increasingly utilised in cisterns. A hydraulic inlet valve utilises the inlet pressure to assist in closing the valve by applying inlet pressure to a relatively large opposite face of the valve member so that a net closing force is achieved. However, hydraulic inlet valves suffer from several disadvantages. One disadvantage is that in the event of a failure of one of the valve components, the hydraulic inlet valve always fails open. That is to say, the valve failure results in flow of water being unable to be stopped. Since the consequence is a flooded bathroom and a substantial loss of water, by far the better arrangement is that the valve, if it were to fail at all, should fail closed. Then the consequence would be that no flow of water could be initiated.
In relation to cistern inlet valves, in addition to failing closed, another desired ligature is quiet operation. Traditionally, inlet valves have been a source of surprisingly substantial noise volumes during their operation. Since the flushing operation is of a relatively short duration, but the re-filling of the cistern with the next flushing water volume takes a substantial time, the noise generated by the inlet valve is present during the entire cistern filling period. This noise is generated by cavitation, turbulence and water tree surface noises (the latter being due to water-air-water interfaces). A substantial marketing advantage is available if a quiet cistern inlet valve can be achieved.